The invention relates to a slip ring and brush assembly, and more particularly, to a wide band DC to UHF module in which a vernier connection comprising an unequal number of slip ring feeds and brush leads is employed.
It can be shown that a slip ring assembly which comprises a single feed wire to the slip ring and a single brush lead will exhibit a reflection coefficient of ##EQU1## In this equation, .theta. equals the ring circumference in radians of transmission line length, and .phi. equals the displacement between the slip ring feed and the brush, also expressed in radians. This equation assumes that the characteristic impedance of the slip ring is twice the characteristic impedance of the feed line.
In many applications, a specific slip ring assembly will require a ring of such large diameter, that when operated at a high frequency, the reflection coefficient given by the equation above will be excessive. In such a situation, the use of multiple feeds is helpful. For example, if four brushes and four slip ring feeds are used, the reflection coefficient would be given by the equation above where .theta. becomes one-fourth of the slip ring circumference. In this way, for a given reflection coefficient and slip ring diameter, the maximum frequency which can be transmitted would be proportional to the number of feeds and brushes.
If a perfect match of the characteristic impedance could be maintained at each slip ring and brush junction, signals of any frequency could be transmitted by a slip ring of any diameter through the use of a sufficiently large number of feeds and brushes. However, since the slip ring feeds must be connected in parallel at the source, and the brush leads must be connected in parallel at the load, then a perfect impedance match would require that the characteristic impedance of the leads to be N times the system impedance, where N is the number of feeds and brushes. Since a proper impedance match requires that the slip ring impedance be twice the impedance of the feed and brush leads, a proper match is impossible where more than a very few feeds are used. Although a reasonable mismatch is acceptable, a point is rapidly reached where the addition of more feeds would exacerbate the mismatch at the ring in an amount sufficient to prevent any improvement in the reflection coefficient.
An improvement in the minimum value of the reflection coefficient may be obtained through the use of an unequal number of brushes and ring feeds. Such an arrangement may be called a vernier connection. It will be noted that, according to equation 1, the reflection coefficient is a periodic function of .phi., reaching a maximum when .phi. equals zero, and becoming zero when .phi. equals .theta./2. When an equal number of feeds and brushes are used, .phi. is the same relative to all of the feeds, and consequently, maximum reflection occurs at each of the ring connections simultaneously with a given slip ring position. Where .phi. equals .theta./2, the reflection is zero.
In the case of a vernier connection, a first number of ring feeds are approximately equally spaced around the circumference of the ring and a second number of brushes are approximately equally spaced around the circumference of the ring. This is to be distinguished from a slip ring construction in which multiple connections of ring feeds and brushes are used for realizing increased current capacity or for reliability based upon reduncancy. In such construction, the ring feeds and brushes are not spaced around the ring and consequently there is no vernier effect. In a true vernier connection, however, where one ring feed is directly under a brush, .phi. equals zero and a maximum reflection occurs, but the other feeds are removed from the brushes by varying amounts. This results in smaller reflections occurring at these other feeds. Since the maximum reflection never occurs on all feeds simultaneously, the overall maximum reflection is reduced, and while the minimum reflection may not be zero, the net result is an averaging of the individual reflections on the ring. Since a limiting factor is the value of maximum reflection which occurs with a given slip ring construction, and the reflection coefficient is a function of frequency, a ring using a vernier connection is able to operate at a higher frequency than a ring using conventional multiple connections. A vernier connection produces a reduction of the maximum reflection coefficient as well as a reflection coefficient which is more nearly independent of shaft rotation due to the averaging effect. No such arrangement is known in the prior art.